1. Field of the Invention
The present invention relates to an apparatus for measuring an exchange force between a surface of a specimen and a probe which is faced to the specimen surface by a very small distance.
2. Description of the Related Art
Heretofore, in many known methods of analyzing solid specimens using an electron beam, the intensity (the number of electrons) and the kinetic energy are adopted as a measure for analysis. Another measure for the investigation is electron spin. There have been proposed several methods of evaluating a microscopic surface magnetism of a solid substance on the basis of the electron spin. For instance, there have been proposed several methods of determining directions of magnetic moments of respective atoms with atomic resolution as illustrated in FIG. 1.
In accordance with recent progress in electronics, a recording density on magnetic recording medium has become higher year after year. FIG. 2 is a chart representing a variation of the recording density in accordance with progress in the magnetic recording medium and various methods of evaluating the surface magnetism. The horizontal axis denotes a time in the Gregorian year, the left-hand vertical axis a linear recording density (cycle/cm), and the right-hand vertical axis represents a resolution of methods of evaluating the surface magnetism in .mu.m and nm. Magnetic recording began in 1900 having a wavelength of 1 mm and have become more and more dense. The linear recording density has been improved in audio magnetic tape, .beta. magnetic tape and VHS magnetic tape. In a recent evaporation tape, a length of one bit is 0.3-0.4 .mu.m. In a modern hard disc, a length of one bit has shortened to 0.16-0.19 .mu.m. By the electron holography, magnetic bits of 0.085 .mu.m were observed on Co-Cr media. The resolution in evaluation methods of surface magnetism has been also improved. The resolution of the Bitter technique has been improved from 1 .mu.m to 0.7 .mu.m, and the resolution of the Kerr effect method has improved from 1 .mu.m to 0.5 .mu.m. The resolution of the spin-polarized scanning electron microscopy (SP-SEM) has improved from 100-200 .mu.m in 1984 to 20 nm in 1994. The magnetic force microscopy (MFM) had a resolution of 100 nm in 1987 and had a resolution of 10 nm in 1988. The electron holography had a resolution of 10 nm in 1991 and the Lorentz microscopy has a resolution of 10 nm now and will have a resolution of 0.7 nm in a near future.
As explained above, the resolution of surface magnetic evaluation has become higher and higher. However, a higher resolution is required on in either basic studies of material properties or engineering, for instance magnetic recording. Hence, it has been earnestly required to develop an evaluation method which can evaluate magnetic properties of a solid surface with an atomic resolution. The inventors of the present application have proposed a spin-polarized scanning tunneling microscopy (SP-STM).
FIG. 3 is a schematic view illustrating an experimental apparatus for proving the utility of SP-STM. In an actual SP-STM, a specimen is made of a magnetic material and a probe is made of gallium arsenide (GaAs). However, in the experimental apparatus, a specimen was made of GaAs and a probe was made of nickel (Ni). This does not cause any problem as long as the principle of the SP-STM is investigated. A single-mode laser diode 1 was used as a linearly polarized light source of about 830 nm in wavelength and about 30 mW in maximum output power. Linearly polarized laser beam was made incident upon a Pockels cell 3 by means of an lens 2. To the Pockels cell 3, was applied a high voltage from an oscillator 4 via a high voltage amplifier 5. Then, an excited circularly polarized laser beam was modulated into right-hand circularly polarization and left-hand circularly polarization at a modulation frequency of about 400 Hz. In this manner, the spin-polarization of excited electrons was changed. The modulated laser beam was made incident upon a specimen 11 as exciting light by means of reflection mirror 6-8, .lambda./4 plate 9 and lens 10. A probe 12 made of a crystal wire of Ni was biased by a DC voltage source 13 was brought into a close proximity of the surface of specimen 11 under the control of a Piezoelectric element 14 such that a tunneling current could flow from the specimen to the probe. The generated tunneling current was detected by a control unit 15, and an output signal of the control unit was supplied to a monitor 16 together with an output signal from the oscillator 4. In this manner, the tunneling current depending upon the spin-polarization of the surface of specimen 11 was detected.
In the above explained SP-STM, the tunneling current produced by the radiation excitation is detected, and thus could not be applied to electrically insulating magnetic materials. The inventors have proposed a possibility of an atomic force microscopy (AMF) which could detect the exchange force between a sample and a probe. Such an atomic force microscopy could be applied to insulating objects.
In the known atomic force microscopy, the measurement is performed within a non-contact region in which the tip of the probe is separated from the specimen surface by a relatively large distance or within a direct contact region in which the tip of probe is brought into contact with the specimen surface. In the measurement within the non-contact region, magnetic forces produced between magnetic dipoles are measured. However, these forces are of a long-range force, and thus it is impossible to realize an atomic resolution. In the measurement within the direct contact region, although it would be possible to evaluate the surface structure with an atomic resolution, it is impossible to measure the exchange force between the specimen and the probe in an accurate manner, because the probe tip is brought into contact with the specimen surface and is influenced by magnetic properties of the specimen surface. Therefore, it is impossible to evaluate inherent magnetism of the specimen surface in an accurate manner.
In order to overcome the above mentioned drawback, the inventors have proposed, in a co-pending patent application, a method of measuring an exchange force between a probe and an electrically conductive or electrically insulating specimen with an atomic resolution.
In this method, in order to measure an exchange force between a specimen and a probe each of which contains localized electrons and at least one of which contains conduction electrons, the specimen and probe are faced to each other by a distance within a close proximity region from a distance at which conduction electron clouds (wave function) begin to be overlapped with each other to a distance at which localized electron clouds (wave function) are not substantially overlapped with each other, and an exchange force between said two substances is measured. The above close proximity region is called RKKY type exchange interaction region.
FIG. 4 is a graph showing variations of force and energy between the specimen and the probe in accordance with a distance therebetween. It should be noted that the force may be derived by differentiating the energy. The RKKY type exchange interaction region is between the contact region in which a direct exchange interaction is taken place and the non-contact region in which an interaction between magnetic dipoles is carried out. In the known atomic force microscope, the direct exchange interaction region or non-contact region is used. In these regions, the force between the specimen and the probe could not be measured with an atomic resolution. It should be noted that in FIG. 4, boundaries between the direct exchange interaction region, RKKY-type exchange interaction region and magnetic dipole interaction region are denoted by broken lines, but in practice, these boundaries could not be determined clearly.
When a specimen and a probe are faced to each other by a distance within the RKKY-type exchange interaction region, an exchange force between the specimen and the probe is of an order of 10.sup.-10 N. Presently available atomic force microscope has a measuring limit of an order of about 10.sup.-12 -10.sup.-13 N. Therefore, the exchange force of an order of 10.sup.-10 N could be measured.
However, if an exchange force within the RKKY-type exchange interaction region is measured using a cantilever of the known atomic force microscope in which the non-contact region is utilized, the probe is brought into contact with the specimen, because a distance between the specimen and the probe could not be controlled precisely. Since a spring constant of the cantilever is very small, when the probe is brought into a close proximity of the specimen, a resilient force of the cantilever might be against a force between the specimen and the probe and the cantilever is attracted to the specimen. When a spring constant of the cantilever is increased, a sensitivity of the cantilever might be decreased largely and the exchange force of an order of 10.sup.-10 N between the specimen and the probe could not be measured precisely.